It is not correct. It should be:I've checked with my teacher long ago and he has confirmed that the phrase is wrong.
The context given in your sentence shows that you are talking about an even that happened in the past without reference to the present. Use the past simple.I checked with my teacher long time ago and he confirmed that the phrase was wrong.
You cannot also say "has confirmed" because once again the context shows that the teacher's confirmation came in the past as well. Here again, the past simple should be used.
- For Teachers